The present invention relates to diffraction gratings in optical communications networks and systems, and more particularly to wavelength division multiplexers utilizing diffraction gratings.
Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Wavelength division multiplexing is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber.
In this document, these individual information-carrying lights are referred to as either xe2x80x9csignalsxe2x80x9d or xe2x80x9cchannels.xe2x80x9d The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a xe2x80x9ccomposite optical signal.xe2x80x9d
The term xe2x80x9cwavelength,xe2x80x9d denoted by the Greek letter xcex (lambda) is used herein in two senses. In the first usage, this term is used according to its common meaning to refer to the actual physical length comprising one full period of electromagnetic oscillation of a light ray or light beam. In its second usage, the term xe2x80x9cwavelengthxe2x80x9d is used synonymously with the terms xe2x80x9csignalxe2x80x9d or xe2x80x9cchannel.xe2x80x9d Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, xcex, and a plurality of n such channels are referred to as xe2x80x9cn wavelengthsxe2x80x9d denoted xcex1-xcexn. Used in this sense, the term xe2x80x9cwavelengthxe2x80x9d may be understood to refer to xe2x80x9cthe channel nominally comprised of light of a range of physical wavelengths centered at the particular wavelength, xcex.xe2x80x9d
A crucial feature of fiber optic networks is the separation of the composite optical signal into its component wavelengths or channels, typically by a wavelength division multiplexer. This separation must occur to allow for the exchange of signals between loops within optical communications networks. The exchange typically occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths.
FIG. 1a schematically illustrates one form of an add/drop system, which typically exists at connector points for the management of the channel exchanges. The exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network. In other words, each composite optical signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop.
A wavelength division multiplexer (WDM) typically performs separation of a composite optical signal into component channels in an add/drop system. Used in its reverse sense, the same WDM can combine different channels, of different wavelengths, into a single composite optical signal. In the first instance, this WDM is strictly utilized as a de-multiplexer and, in the second instance, it is utilized as a multiplexer. However, the term xe2x80x9cmultiplexerxe2x80x9d is typically used to refer to such an apparatus, regardless of the xe2x80x9cdirectionxe2x80x9d in which it is utilized.
FIG. 1a illustrates add/drop systems 218 and 219 utilizing wavelength division multiplexers 220 and 230. A composite optical signal from Loop 110 (xcex1-xcexn) enters its add/drop system 218 at node A (240). The composite optical signal is separated into its component channels by the WDM 220. Each channel is then outputted to its own path 250-1 through 250-n. For example, xcex1 would travel along path 250-1, xcex2 would travel along path 250-2, etc. In the same manner, the composite optical signal from Loop 150 (xcex1xe2x80x2-xcexnxe2x80x2) enters its add/drop system 219 via node C (270). The signal is separated into its component channels by the WDM 230. Each channel is then outputted via its own path 280-1 through 280-n. For example, xcex1xe2x80x2 would travel along path 280-1, xcex2xe2x80x2 would travel along path 280-2, etc.
In the performance of an add/drop function, for example, xcex1 is transferred from path 250-1 to path 280-1. It is combined with the others of Loop 150""s channels into a single new composite optical signal by the WDM 230. The new signal is then returned to Loop 150 via node D 290. At the same time, xcex1xe2x80x2 is transferred from path 280-1 to path 250-1. It is combined with the others of Loop 110""s channels into a single new composite optical signal by the WDM 220. This new signal is then returned to Loop 110 via node B (260). In this manner, from Loop 110""s frame of reference, channel xcex1 of its own signal is dropped to Loop 150 while channel xcex1xe2x80x2 of the signal from Loop 150 is added to form part of its new signal. This is the add/drop function.
FIG. 1b illustrates a second form by which add/drop systems 218 and 219 may be configured. In FIG. 1b, each WDM is optically coupled to a first plurality of paths through which channels are outputted and to a second plurality of paths through which signals are inputted. For instance, the paths 250-1, 250-2, . . . , 250-n are utilized to output signals comprising wavelengths xcex1, xcex2, . . . , xcexn, respectively, from the WDM 220 and the paths 251-1, 251-2, . . . , 251-n are utilized to input signals comprising such wavelengths to the WDM 220. Likewise, as shown in FIG. 1b, the paths 280-1, 280-2, . . . , 280-n are utilized to output signals xcex1xe2x80x2, xcex2xe2x80x2, . . . , xcexxe2x80x2n (comprising the physical wavelengths xcex1, xcex2, . . . . , xcexn) respectively, from the WDM 230 and the paths 281-1, 281-2, . . . , 281-n are utilized to input signals comprising such wavelengths to the WDM 230.
FIGS. 2a and 2b illustrate a top view and side view, respectively, of a prior-art grating-based WDM. In the WDM 200, a concave reflection-type holographic grating 202 is disposed upon a substrate plate or block 201 comprised of a material with low thermal expansion. The grating 202, which comprises a portion of a spherical surface 206 centered at point 210, receives a wavelength-division multiplexed composite optical signal 211 input to the WDM 200 from an input fiber 204. The composite optical signal 211 is comprised of a plurality of individual channels, xcex1, xcex2, . . . . The concave grating 202 diffracts, reflects, focuses and spatially disperses each of these individual channels according to its respective wavelength such that each channel is directed to exactly one of a plurality of output fibers 209a-209b. For instance, referring to FIG. 2a, if input signal 211 is comprised of two channels, namely channel xcex1 (207a) and channel xcex2 (207b), with xcex1 greater than xcex2, then, upon back-diffraction from grating 202, the xcex1 channel (207a) and the xcex2 channel (207b) are focused onto the end of fiber 209a and fiber 209b, respectively. input fiber 204 and the plurality of output fibers 209a-209c are disposed within an array 205 of fibers. The end faces of the fibers in array 205 are disposed along or parallel to a plane 208 which makes an angle of 60xc2x0 with the line 203 that is normal to the grating 202 at the center of the grating 202. With this disposition, the grating 202 diffracts light according to the Littrow configuration, in which the angles of incidence and diffraction are approximately equal. FIG. 2b shows a side view of the prior art apparatus taken parallel to the fiber 204. FIG. 2b shows that the fibers are directed towards the grating vertex and are at an angle to the grating dispersion plane 215. The input fiber 204 and the output fibers 209a-209c each make the same angle xcfx86 (taken without regard to algebraic sign) with respect to the dispersion plane 215 and the input fiber 204 makes an angle of 2xcfx86 with respect to the plane of the output fibers. With channels spaced at 0.33 nm, fiber-to-fiber losses within the WDM 200 can be maintained at less than 1 dB and ultra-low crosstalk can be maintained.
For use in commercial optical communications systems, the WDM must limit polarization-dependent loss introduced by the diffraction grating. Polarization-dependent loss results in insertion loss, which compromises the reliability of data carried by the channels.
Accordingly, there exists a need for an improved grating-based wavelength division multiplexer (WDM). The grating-based WDM should provide very low polarization-dependent loss. The present invention addresses such a need.
The present invention provides an improved wavelength division multiplexer (WDM). The WDM includes a birefringent walk-off plate; a reciprocal optical rotator optically coupled to the birefringent plate; and a diffraction grating optically coupled to the reciprocal optical rotator at a side opposite to the birefringent plate. This grating-based WDM provides very low polarization-dependent loss, and therefore, greatly reduces insertion loss. This increases the reliability of the data carried by the channels.